Here is an altar that you will not often get a chance to see in St. Peter’s Basilica: the altar of Saint Petronilla. It’s in the back right corner, which is ordinarily closed off to the public; usually there are a bunch of stacked chairs and other equipment back in that area. Priests can celebrate Mass there in the morning, though I understand that there are usually certain ones who use this altar on a daily basis (i.e. it’s virtually reserved, at least for part of the 7-8:30am daily Mass period). Here is a photo:

St. Petronilla was thought to be St. Peter’s daughter (her name basically means, “Little Peter”). It seems that historians are in disagreement about that. Read more about her here. In any case, she died a martyr, and her relics are inside the altar: